1. Field of the Invention
The present invention relates to a pressurized internal circulating fluidized-bed boiler, and more particularly to a pressurized internal circulating fluidized-bed boiler for use in a pressurized fluidized-bed combined-cycle electric generating system in which a fuel such as coal, petro coke, or the like is combusted in a pressurized fluidized bed and an exhaust gas produced by the combusted fuel is introduced into a gas turbine.
2. Description of the Prior Art
Efforts to reduce the emission of carbon dioxide from various sources are important in view of environmental damages that are being caused by air pollution which appears to be more and more serious on the earth. It is conjectured that coal will have to be relied upon as a major energy resource because greater dependency on nuclear and oil energies is not favorable at present. To suppress carbon dioxide emission and provide a substitute for oil and nuclear power, there has been a demand for a highly efficient, compact electric generating system which is capable of utilizing coal combustion to generate a clean energy.
To meet such a demand, atmospheric fluidized-bed boilers (AFBC) capable of burning coals of different kinds for electric generation have been developed because a stable energy supply cannot be achieved by pulverized coal boilers which pose a limitation on available coal types.
However, the atmospheric fluidized-bed boilers (AFBC) fail to perform the functions that have been expected. In addition, since only steam turbines can be combined with the atmospheric fluidized-bed boilers, there are certain limitations on attempts to increase the efficiency and energy output of the atmospheric fluidized-bed boilers. These disadvantages of the atmospheric fluidized-bed boilers have directed research and development trends toward pressurized fluidized-bed boilers (PFBC) that make it possible to construct combined-cycle electric generating systems with gas turbines.
One combined-cycle electric generating system which incorporates a conventional pressurized fluidized-bed boiler will be described below with reference to FIG. 15 of the accompanying drawings.
As shown in FIG. 15, a pressure vessel 30 houses therein a combustor 31 which is supplied at its bottom with air under pressure from a compressor 32. The pressure vessel 30 also accommodates a bed material storage container 33 which communicates with the combustor 31 to allow a fluidizing medium to move between the combustor 31 and the bed material storage container 33. The combustor 31 has a heat transfer tube 34 disposed therein which is connected to a steam turbine 35.
A dust collector 36 is positioned adjacent and connected to an upper portion of the combustor 31. Dust particles contained in an exhaust gas discharged from the combustor 31 are removed by the dust collector 36. Thereafter, the exhaust gas is supplied to a gas turbine 37.
Operation of the pressurized fluidized-bed electric generating system shown in FIG. 15 is as follows:
Coal is roughly crushed and supplied, together with a desulfurizer such as limestone, to the combustor 31. In the combustor 31, there is generated a fluidized bed, about four meters high, by air supplied under pressure from the compressor 32, the fluidized bed being composed of a fluidizing medium comprising a mixture of a bed material, coal, a desulfurizer, ash, etc. The coal is mixed with air in the fluidized bed, and combusted under pressure. Heat generated in the fluidized bed is recovered as steam by the heat transfer tube 34 in the fluidized bed. The steam is supplied from the heat transfer tube 34 to the steam turbine 35, which is rotated to actuate an electric generator coupled thereto.
The exhaust gas produced by the combustion of the coal in the combustor 31 is supplied to the dust collector 36, which removes dust particles from the exhaust gas. The exhaust gas is then supplied from the dust collector 36 to the gas turbine 37, which actuates the compressor 32. Residual energy contained in the exhaust gas actuates an electric generator coupled to the gas turbine 37.
The exhaust gas discharged from the outlet of the gas turbine 37 is supplied to a denitrification unit 45 which reduces the NOx content and smoke dust in the exhaust gas. The waste heat of the exhaust gas is then recovered by an economizer 38. Thereafter, the exhaust gas is discharged from a smoke stack 39.
The pressurized fluidized-bed electric generating system shown in FIG. 15 is controlled to meet a load imposed thereon by varying the height of the fluidized bed in the combustor 31. More specifically, the fluidizing medium is drawn from the combustor 31 into the bed material storage container 33 to expose heat transfer surfaces of the heat transfer tube 34, thereby controlling the heat generation to meet the load. When the heat transfer surfaces of the heat transfer tube 34 are exposed, the heat transfer coefficient thereof is lowered, and hence the amount of heat recovered is lowered. Since the exhaust gas emitted from the fluidized bed is cooled by the exposed heat transfer surfaces, the temperature of the exhaust gas supplied to the gas turbine 37 is lowered, thus reducing the output energy of the gas turbine 37. However, the above control process is disadvantageous in that the bed material storage container 33 is necessary to withdraw and return the high-temperature fluidizing medium from and into the combustor 31, it is not easy to withdraw and return the fluidizing medium at high temperature and pressure, and agglomeration tends to occur when the fluidizing medium particles of high heat density are taken into and out of the bed material storage container 33.
Furthermore, since the pressurized fluidized-bed boiler is under pressure, the heat transfer tube 34 in a splash zone of the fluidized bed is more subject to wear than that in the atmospheric fluidized-bed boilers. Another problem is that an large amount of carbon monoxide is produced because the exhaust gas emitted from the fluidized bed is cooled by the heat transfer tube 34 and the exhaust gas remains in the fluidized bed for a short period of time as the height of the fluidized bed is reduced.
As described above, limestone is mixed with the fluidizing medium for desulfurization in the conventional pressurized fluidized-bed electric generating system shown in FIG. 15. However, the limestone wears rapidly, and is scattered as ash from the dust collector 36 without sufficiently contributing to the desulfurizing action. The conventional pressurized fluidized-bed electric generating system fails to achieve a high desulfurization rate that are required by thermal power plants. If the desulfurization rate is increased, then the conventional pressurized fluidized-bed electric generating system produces a vast amount of scattered ash.